1. Field of the Invention
This invention relates to gas discharge lamp starters and more particularly to a solid state fluorescent lamp starter.
2. Description of the Prior Art
Fluorescent lamps are well-known in the art and are used for a variety of types of lighting installations. Such lamps are characterized as low pressure discharge lamps and include an elongated envelope whose interior wall is coated with a phosphor such as calcium halophosphate, zinc silicate or calcium tungstate, and an electrode structure at each end of the envelope. The envelope also contains a quantity of an ionizable material such as mercury, and a fill gas at low pressure, generally in the range of one to five mm. Hg. The fill gas may be a single gas or a mix of gases such as argon, krypton and neon.
In pre-heat type fluorescent lamps, the electrode at each end of the envelope is an electric filament. Lamp circuitry is utilized which provides a small current through the electrodes which in turn heats the filaments.
The electrode filament is usually of coiled tungsten wire having an emissive coating such as barium oxide that produces electrons when heated above 800 degrees C..degree.. The emitted electrons accelerated by the electric field in the vicinity of the cathode electrode, bombard the vaporized mercury in the vicinity of that electrode.
A voltage, supplied by a ballast across the lamp, from the electrode at one end of the lamp to the electrode at the other end of the lamp further accelerates the free electrons, ionizing additional mercury atoms. As a result of this ionization and subsequent recombination of ions and electrons, ultraviolet radiation is produced. The phosphor on the wall of the lamp generates visible radiation in response to the impinging ultraviolet radiation.
Conventional flourescent lamp starters provide a source of heating current to the filaments by connecting the two filaments, one at each end of the lamp, in series through a switching device which opens after a predetermined time interval when the coils have been heated to a proper temperature for electron emmission.
Prior art starters have been complex and often expensive to manufacture. They may include, a glow bottle and capacitor, or may be constructed as a thermal switch. Some more recent devices have been constructed with numerous solid components on a printed circuit board.
The glow bottle device is essentially a combination of a neon lamp and a bimetallic contact wired across the neon lamp electrodes. When the bimetallic contact is cool, the contacts remain open and the neon lamp can generate light and heat. Upon application of current to the starter circuit, the neon lamp portion of the glow bottle device begins to glow and eventually generates enough heat to cause the bimetallic contacts to come together shorting out the neon lamp. This allows a high enough current flow to heat up the cathodes. Since the neon lamp is shorted, it ceases to glow and generate heat thus allowing the bimetallic contacts to cool. After the bimetallic contacts have cooled sufficiently, they open and interrupt the circuit. Upon the opening of the circuit and in accordance with Lenz's law, an inductive voltage spike is generated by the inductor.
The inductive spike can function in one of two ways. First, if it is very large it can jump across both electrodes in the fluorescent lamp and cause ionization of the gas within the envelope which will then lead to starting of the lamp.
The second mode of operation occurs if a ground plane, usually the lamp fixture, is located sufficiently close to the lamp. Upon application of the large voltage pulse, atoms of gas within the envelope near the electrode coupled to the choke are ionized. This ionization eventually permits the lamp to start. In some instances, repeated cycling is necessary to sufficiently ionize the mercury in the vicinity of the electrode.
One disadvantage of the above describe prior art starters is that unrealiable mechanical devices are used which are bulky in nature and relatively expensive to manufacture. Furthermore, due to the inductive reactance of the choke, in order to develop a sufficient pulse the bimetallic contacts should open on a portion of high flow of the sinusoidal input current since the voltage in an inductor is equal to -Ldi/dt, where L is the inductance, and di/dt represents the current flow over time. The operation of the glow bottle device, however, is not synchronized with commonly used sinusoidal input current and therefore several starting attempts are often needed before the lamp lights. This leads to the familiar flickering of fluorescent lamps before they light.
Various solid state devices have been proposed for starting fluorescent lamps. They, however, have been multi-component devices and physically at least as large as a conventional glow-bottle starter.